Liquid-crystal displays (LCDs) are commonly used in projection displays for large screen televisions and monitors. In these LCD-based projection systems, a high power beam of light is passed through a polarizer before being incident on a LCD panel. The LCD panel controls the polarization of the incident light pixel-by-pixel and redirects it towards the corresponding polarizer/analyzer, which then redirects light having the proper polarization to a projection lens that projects an image onto a screen.
One particularly successful LCD-based projection system is a WGP-based LCoS microdisplay system, which uses both wire grid polarizers (WGPs) and liquid crystal on silicon (LCoS) panels. This microdisplay system, which has been proven to exhibit both high resolution and high image contrast when compared to other microdisplay technologies such as transmissive liquid crystal (xLCD), digital light processor (DLP), and direct-view LCD, typically uses three or more microdisplay panels (e.g., one for each primary color band) to improve on-screen brightness.
Referring to FIG. 1, a conventional 3-panel WGP-based LCoS microdisplay system is shown. The microdisplay system includes a light source 5, which for example is a high-pressure discharge lamp, and a light rod 7. The light rod 7 homogenizes the cone of light produced by the light source 5 to ensure a spatially uniform light distribution. Optionally, the light rod 7 is a polarization conversion light pipe (PCLP) for producing linearly polarized light. A first lens 8a passes the light from the light pipe 7 to a first folding mirror 9, which directs the light to a first dichroic filter 10. The dichroic filter 10 separates out the blue light from the remaining light, and directs the blue light via second 8b and third 8c lenses, and second 17 and third 16 folding mirrors to a first LCoS display panel 20a. The remaining light, which is transmitted through the dichroic filter 10, is directed via fourth and fifth lenses 8d and 8e and a fourth folding mirror 11 to a second dichroic filter 12. The second dichroic filter 12 separates the remaining light into green and red light, the former of which is directed to a second LCoS display panel 20b and the latter of which passes to a third LCoS display panel 20c. 
Prior to reaching each LCoS display panel 20a, 20b, and 20c, the incident light first passes through a WGP 15, 14, and 13 and a trim retarder compensator 21a, 21b, and 21c, respectively. Each WGP 15, 14, and 13 is a polarizer/analyser formed from a plurality of parallel micro-wires that transmits light having a polarization orthogonal to the direction of the parallel micro-wires and reflects light having a polarization parallel to the direction of the wires; e.g., if the polarizers are designed to pass horizontal or P-polarized light, as illustrated in FIG. 1, the micro-wires will be perpendicular to the plane of FIG. 1. Each LCoS panel 20a, 20b, and 20c alters the polarization of the linearly polarized incident light pixel-by-pixel and reflects the modulated light back to the corresponding WGP 15, 14, and 13. Since each WGP 15, 14, and 13 is orientated at approximately ±45° with respect to the principal direction of light propagation, in addition to serving as a polarizer/analyzer, each WGP 15, 13 and 14 also serves as a beamsplitter for separating the incoming light from the outgoing light by steering or deflecting the light reflected from the each LCoS panel along an output optical path orthogonal to the incoming optical path. More specifically, each WGP 15, 14, and 13 reflects S-polarized light, e.g., polarized light rotated by 90° by pixels in an ON state, to the X-cube 19. The X-cube 19 aggregates the images from each of the three color channels and, via the projection lens 18, projects the final image onto a large screen (not shown). Optionally, each color channel further includes a pre-polarizer (not shown) and/or a clean-up analyzer (not shown), which for example, may include one or more WGPs and/or dichroic sheet polarizers.
The reflective LCoS panels 20a, 20b, and 20c, hereinafter generally referred to as LCoS panels 20, may be either twisted nematic, e.g. 45.degree. twist (45TN), or vertically-aligned nematic (VAN-mode) panels, which get switched (or relaxed) to near homeotropic orientation. Other LC-modes in reflective LCOS and transmissive xLCD, i.e. bend-aligned nematic or pi-cell, also require trim retarders, if the LC-technology employs a dark-state director orientation near the homeotropic alignment. A VAN-mode cell on a reflective substrate is functionally equivalent to a pi-cell in transmission mode, i.e. both operate as electrically controllable birefringence for gray-scale with viewing angle symmetry about an axis orthogonal to the LC tilt-plane.
In homeotropic alignment the LC uniaxial positive molecules are oriented normal to the device plane. The dark, or OFF state may be a switched, or voltage-driven state or a relaxed state where no or little voltage is applied, depending on LC modes. In most applications, a true homeotropic orientation in the dark state is not suitable, i.e. a pre-tilt is required to provide consistent and faster switching behavior. Moreover, true homeotropic orientation in the dark state may not be available due to a lack of high voltage supplies in 45TN panels wherein the dark state requires the application of the electric field to the LC film, or due to boundary LC layers being anchored by alignment surface effects. As a consequence, the display panels in the dark state exhibit both an in-plane and an out-of-plane residual retardation component, i.e. A-plate and C-plate components, respectively. Due to the use of positive-only uniaxial LC in LCD panels, the c-plate component is always positive, thereby adding to the net panel retardance at off-axis illumination.
The trim retarder compensators 21a, 21b, and 21c, hereinafter simply referred to as trim retarders (TR) 21, are compensating elements used to improve the contrast performance level of the microdisplay system, which is otherwise limited by the residual birefringence of the LCoS panels in the dark, e.g., off state. In particular, each trim retarder 21 introduces a phase retardance that cancels the retardance resulting from the inherent birefringence of the corresponding LCoS panel. The term ‘retardance’ or ‘retardation’, as used herein, refers to linear retardance magnitude as opposed to circular retardance magnitude, unless stated otherwise. Linear retardance is the difference between two orthogonal indices of refraction times the thickness of the optical element. Linear retardance causes a phase difference between two orthogonal linear polarizations, where one polarization is aligned parallel to the extra-ordinary axis of the linear retarder and the other polarization is aligned parallel to the ordinary axis of the linear retarder. In contrast, circular retardance causes a relative phase difference between right- and left-handed circular polarized light.
Linear retardance may be described as either in-plane or out-of-plane retardance. In-plane retardance (IPR), expressed as optical path length difference, refers to the difference between two orthogonal in-plane indices of refraction times the physical thickness of the optical element. Out-of-plane retardance refers to the difference of the index of refraction along the thickness direction (z direction) of the optical element and one in-plane index of refraction, or an average of in-plane indices of refraction, times the physical thickness of the optical element. Normal incidence rays in a cone bundle see only in-plane retardance, whereas off-axis rays including oblique rays (i.e. non-normal but along the principal S- and P-planes) and skew rays (i.e. non-normal and incident away from the principal S- and P-planes) experience both out-of-plane retardance and in-plane retardance.
In the absence of trim retarders 21, the P-polarized light that illuminates each microdisplay panel in the dark (off) state is slightly elliptically polarized upon reflection due to the residual birefringence of the LCoS panels 20. When the elliptically polarized light, which contains both a P- and an S-component, is transmitted to the corresponding WGP 15, 14, 13, the S component is reflected to the X-cube 19 thus allowing dark state light leakage onto the large screen and limiting the contrast of the projection system.
The TR-compensated LCoS panel comprising the TR 21 and LCoS panel 20, also referred to as the imager assembly, is schematically shown in FIG. 2 in the context of a single-channel optical system 25. An input ray 26 within the defined cone angle range is preferentially polarized by a pre-polarizer 23, its polarization contrast further enhanced by a polarization beam splitter (PBS) 14, such as a WGP, this linearly polarized ray is pre-conditioned to be an elliptical polarized light upon transmission through the retarder compensator 21 and the residual birefringence in the LCoS panel 20 substantially undoes the ellipticity such that on double pass through the cascade of retarder compensator 21 and LCoS 20 devices, a linear polarization light output is obtained, which is ideally returned to the illumination system as 27 and not being deflected to the projection lens/screen by the PBS 14.
The origin of the residual retardance of an LCoS panel in a dark state is illustrated in FIG. 3, schematically showing the dark-state LC molecules orientation in one cell 70 of a VAN-LCoS display panel 20. The VAN-LCoS cell 70 comprises a substrate 71 and a cover glass 72, sandwiching a LC gap, 73 of a vertical size d. The cell 70 is filled with a nematic liquid crystal with its LC director 74 aligned at a slight polar angle tilt, θc, 75 from the device normal, i.e. the Z-axis direction, and the projection of the LC director onto the plane of device is oriented at an azimuthal offset φc 76 from the X-axis. The in-plane Γa LC and out-of-plane Γc LC retardance of the LCoS cell are given by a known quadratic effective index formula.
For a very small pre-tilt angle (<<10°) and low birefringence, the in-plane and out-of-plane retardances are approximately the squares of sine and cosine of the pre-tilt angle multiplied by the LCoS cell retardance Δn·d, respectively, where the birefringence Δn=(ne−no) is the difference between the extraordinary refractive index ne and the ordinary refractive index no of the LC material in the LCoS cell 70.
The use of trim retarders 21 improves the contrast level by providing in-plane retardance that compensates for the retardance resulting from the residual birefringence in the respective LCoS panels 20. More specifically, the trim retarders 21 can be selected to have the same single-pass IPR as the corresponding LCoS displays 20, and oriented such that their slow axes are at orthogonal azimuthal alignment to the slow axes of the LCoS panels 20,
while their fast axes are at orthogonal azimuthal alignment to the fast axes of the LCoS panels 20, resulting in a configuration conventionally termed “crossed axes” configuration. This TR/LCoS panel configuration is illustrated in FIG. 4 by arrows 61 and 63, with the arrow 61 representing the LCoS 20 slow axis, and the arrow 63 representing the TR 21 slow axis. The terms slow axis (SA) and fast axis (FA), as used herein, refer to the two orthogonal birefringent axes when the linear retardance is measured at normal incidence. Notably, the SA and FA directions may change with off-axis illumination.
The LCoS SA 61 is shown in the second quadrant, with an azimuthal angle of 62, relative to the +X-axis; a right-hand XYZ coordinate system is assumed, RH-XYZ, with the z-axis directed normally to the LCoS/TR plane, and the x-axis directed along the polarization direction of the incident P-polarized light, with the y-axis directed along the polarization direction of the S-polarized light; this relationship between the three axes of the XYZ coordinate system and the polarization orientation of the incident and reflected light is assumed throughout this document. The SA 61 of the LCoS panel is typically oriented to be substantially parallel to the bisector of the S- and P-axes. Notably, orienting the slow axis of the VAN-LCoS at ±45° or ±135° is important for the VAN-LCoS panel to function as an efficient electrically-controlled birefringence (ECB) device, providing a crossed polarization conversion of light according to equation (1):
                                          I                          (                              output                ⁢                                                                  ⁢                crossed                ⁢                                                                  ⁢                polarization                            )                                            I                          (                              input                ⁢                                                                  ⁢                linear                ⁢                                                                  ⁢                polarization                            )                                      =                              [                                          sin                ⁡                                  (                                                                                    2                        ⁢                                                                                                  ⁢                                                  Γ                          eff                                                                    λ                                        ⁢                    π                                    )                                            ⁢                              sin                ⁡                                  (                                      2                    ⁢                                                                                  ⁢                    ϕ                                    )                                                      ]                    2                                    (        1        )            
where Γeff is the effective single-pass voltage-dependent retardance, in length units, as seen by the incident ray, λ is the illumination wavelength, and φ is the azimuthal orientation angle of the slow-axis relative to the P-polarization. In this configuration, the VAN-LCoS in an on-state functions approximately as a quarter-waveplate retarder in a single pass.
Once the slow axes 61, 63 of the trim retarders 21 and LCoS panels 20 are configured at orthogonal azimuthal orientations, a component of the incident light polarized along the SA 63 of the TR 21 will alternately experience a larger delay when propagating through the TR 21, and a smaller delay when propagating through the LCoS panel 20; conversely, a component of the incident light polarized along the FA of the TR 21, which is directed along the LCoS SA 61, will alternately experience a smaller delay when propagating through the TR 21, and a larger delay when propagating through the LCoS panel 20. If the one-way retardance of the LCoS 20 is equal to that of the TR 21, the net effect is a zero relative delay for the two orthogonal components of the incoming polarization, and as a result, an unchanged polarization of the incident light after propagation through the TR/LCoS assembly 20, 21; i.e., the output light has the same polarization as the incident light. The corresponding WGP 14 and/or an optional clean-up polarizer then rejects the output light so that the dark-state panel leakage does not appear on the screen. Since the trim retarder 21 does not alter significantly the throughput of the LCoS panel on-state, the resulting sequential contrast (full on/full off) is substantially improved.
In addition to providing in-plane retardance, it is common for trim retarders 21 to also provide out-of-plane retardance to increase the field of view. More specifically, it is common for trim retarders to include both an A-plate compensation component for compensating the in-plane retardance and a -C-plate compensation component for compensating the out-of plane retardance. Optionally, the trim retarders 21 also include an O-plate component. An A-plate is an optical retarder formed from a uniaxially birefringent material having its extraordinary axis oriented parallel to the plane of the plate. A C-plate is an optical retarder formed from a uniaxially birefringent material having its extraordinary axis oriented perpendicular to the plane of the plate, i.e. parallel to the direction of normally incident light. A -C-plate exhibits negative birefringence. An O-plate is an optical retarder formed from a uniaxial birefringent element having its extraordinary axis, i.e., its optic axis or c-axis, oriented at an oblique angle with respect to the plane of the plate.
As discussed above, the trim retarder 21 ideally provides an A-plate retardance that matches the in-plane retardance of the corresponding LCoS panel 20 in the off-state. In practice, however, the A-plate retardance of both the LCoS panels 20 and the trim retarders 21 tends to vary within each component due to manufacturing tolerances in device thickness and material birefringence control, as well as due to operational drifts (temperature, mechanical stress etc). As a result, to ensure adequate compensation it is common to provide a higher A-plate retardance in the trim retarders 21 than that exhibited by the LCoS panels 20. For example, a trim retarder with an A-plate retardance of 5 nm is often provided to compensate for a vertical aligned nematic (VAN) LCoS exhibiting a 2 nm A-plate retardance at the same wavelength λ.
As is known to those skilled in the art, this mismatch in A-plate value requires offsetting of the SA of the trim retarder 21, relative to the crossed axes orientation 63 described above, and the optimal contrast is obtained by deviating from the crossed axes configurations. In other words, the trim retarder is mechanically ‘clocked-in’ by rotating its azimuth orientation away from the crossed-axes configuration by an angle φob that is referred to as an over-clocking angle. When the slow and fast axes of the VAN-LCoS panel bisect the S- and P-polarization planes, as discussed above, the over-clocking angle, φob, of a higher IPR value trim retarder is calculated from the following equation:
                              ϕ          ob                ≈                                            cos                              -                1                                      ⁡                          (                              [                                                      Γ                    aLC                                    /                                      Γ                    aTR                                                  ]                            )                                2                                    (        2        )            
where ΓaTR is the trim retarder A-plate retardance and ΓaLC is the LCoS A-plate retardance, with ΓaTR>ΓaLC.
Referring to Table 1, the calculated over-clocking angles for trim retarders providing 2 to 10 nm A-plate retardance for compensating an LCoS panel exhibiting 2 nm A-plate retardance are shown. Both positive and negative azimuthal offsets are given. In addition, two more azimuthal locations are found in the opposite quadrant (i.e., the listed over-clocking angles ±180°).
TABLE 1Approximate over-clocking angles of the trim retardercompensator/VAN-LCoS pair from the nominal crossed-axesconfiguration.Over-clocked angle fromΓa(TR)nominal crossed axes203±24.14±30.05±33.26±35.37±36.78±37.89±38.610±39.2
The mechanical rotation of a discrete trim retarder compensator relative to an LCoS panel as a means of optimizing the LCoS panel contrast is the prevalent assembly methodology in the LCoS display industry, having as large as 20% in-plane retardance distribution from part to part over a large batch of wafers. The active mechanical alignment of each LCoS-TR pair, referred to as the mechanical ‘clocking’, has the required angle granularity to always clock in any given panel.
FIGS. 1 and 2 illustrate a conventional arrangement of a TR/LCD panel assembly, also referred to as the imager assembly, wherein the TR 21 and the LCD panel 20 are separately fabricated elements that are disposed next to each other and held together by a mechanical means. Such an imager assembly, which provides the TR rotation capability for active mechanical clocking of the TR position while protecting the LCoS panel from outside dust, is disclosed in a pending U.S. patent application Ser. No. 11/358,605, which is assigned to the assignee of the instant application. Although the imager assembly simplifies the active mechanical clocking of the TR/LCD panel pair, the associated individual active rotational adjustment of each imager assembly is still time and resource consuming.
An alternative arrangement would be to provide one-piece imager assemblies wherein an LCoS panel is integrated with a TR compensator, as described e.g. in a pending US Patent application 2005/0128391 assigned to the assignee of the current application. Advantageously, this LCoS panel/TR integration could be performed at a wafer level at the same manufacturing step where the LCoS panels are produced, resulting in one-step manufacturing of a plurality of the imager assemblies from a single compensated wafer. However, due to the variations in the residual IPR of the individual LCoS panels from the wafer, this approach would result in inaccurate retardance compensation for at least some of the imager assemblies, and will lower the production yield of high-contrast imager panels.
It is therefore desirable to provide a method for compensating the residual in-plane retardance of the LCoS display panels that does not require the individual mechanical LCoS panel-trim retarder alignment, or clocking, while providing high system contrast.
Accordingly, an object of the present invention is to provide a TR-LCD panel assembly that does not require the step of active mechanical clocking for providing high system contrast.
Another object of the present invention is to provide a method of electronic tuning of the TR-LCD panel polarization alignment for providing a high system contrast.
Another object of the present invention is to provide a method for electronic compensation of the residual in-plane retardance of the LCD panel.
Another object of the present invention is to provide a method of wafer-level manufacturing of the LCD panel-trim retarder assemblies that are suitable for electronic contrast adjustment.
Another object of the present invention is to provide a method for electronic tuning of the image contrast in LCD-based image forming devices.